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"Snowball Earth"

Geologic and paleomagnetic evidence has indicated that a series of intense ice ages occurred during the Proterozoic period of Earth history (roughly 550-2500 million years ago). The photograph above shows distinctive carbonate crystal fans from Brazil deposited during the aftermath of a large glaciation. During the largest of these ice ages, glacial deposits are present on continents that were near the equator, leading many scientists to speculate that a runaway albedo effect (ice reflects more sunlight than rock, diminishing the amount of warming light that is absorbed) led to a completely ice-encrusted planet that has been dubbed "Snowball Earth". This latter theory is particularly controversial, and if correct would have very significant implications for the survival and evolution of life on Earth. Unfortunately, there has thus far been little direct evidence of environmental conditions during these low-latitude glaciations. For more details, see the popular book Snowball Earth by Gabrielle Walker.

Alison Olcott, a PhD student at USC, has been working with Frank Corsetti and myself to use organic molecular fossils ("biomarkers") to provide some more direct evidence of paleoenvironmental conditions. For example, if photosynthetic pigments like chlorophyll are present, we might infer that sunlight was reaching the surface ocean and hence it was not covered by a thick icecap. Answering these and other questions should help us understand how the intense glaciations of the Proterozoic affected the evolution of life.